The present invention relates to an apparatus for line- alternating interpolation of chroma signals, and particularly to an apparatus for line-alternating interpolation of chroma signals in a muse decoder for a high definition television, wherein a color difference signal component which is not yet transmitted on a current scanning line can be obtained by using input color difference signal components of a plurality of peripheral scanning lines, which are offset between lines and line-alternated.
Current television systems differ in their number of scanning lines, aspect ratios, scanning methods, and the like. Each country specifies its standard television system method for displaying the image signal on the screen, e.g., NTSC, PAL, and SECAM systems, etc. Also, current television systems have limited resolution, which has led to developments being advanced toward a system with resolution superior to that of current television systems.
A recently developed television system having excellent resolution is a high definition television (HDTV) system referred to as a multiple sub-Nyquist sampling encoding (MUSE) system, developed in Japan.
Among the foregoing television systems, the NTSC system transmits a luma signal Y and two color difference signals R-Y and B-Y loaded on every line. Although the usable bandwidth in the NTSC system is 6 MHz, in practice, the broadcasting station uses 4.5 MHz as the transmission bandwidth. This is because, among the bandwidths of the image signals, that of the chroma signal is 4.2 MHz and the audio signal's bandwidth is 4.5 MHz. However, in the MUSE system, since the bandwidth of the image signal is made to be 20 MHz, but is compressed to 8 MHz at the broadcasting station before transmission, the luma signal Y and a single color difference signal are transmitted in every line in order to compress the data to be transmitted.
Accordingly, to recreate a better high definition image, it is necessary to interpolate the untransmitted signal into each line. Thus, development of an interpolating circuit for the MUSE system is being actively carried out.
FIG. 1 illustrates a conventional apparatus for line-alternating interpolation of chroma signals.
In FIG. 1, through an input terminal IN, a first delay device 1 receives an external digital chroma signal and delays it for a predetermined time, and then supplies the delayed digital chroma signal as an output signal to a second delay device 2. Second delay device 2 receives the delayed digital chroma signal from first delay device 1 and delays it for the same time as that of first delay device 1, and then supplies the further delayed signal to one input terminal of an adder 3. Adder 3 is supplied with the digital chroma signal delayed by both first and second delay devices 1 and 2 through one input, and with the undelayed digital chroma signal through the other input. Therein, both digital chroma signals are added together. At this time, in view of the digital chroma signal, the digital chroma signals of three lines are placed in parallel on the same time axis.
The resultant sum of adder 3 is obtained by adding together digital chroma signals having a difference of two lines. In the MUSE system as described above, the luma signal Y and one of the two color difference signals (R-Y or B-Y) are loaded on each line, and then transmitted. As a result, the same color difference signals are added. A first multiplexer 4 is supplied with both the resultant sum from adder 3 through one input terminal A, and the output signal of first delay device 1 through the other input terminal B. Meanwhile, a second multiplexer 5 receives the output signal of first delay device 1 through one input terminal A and the resultant sum of adder 3 through the other input terminal B. Also, first and second multiplexers 4 and 5 respectively output one signal of the two received inputs in accordance with a line-alternating signal LAS.
An example of the above process is described below to further understanding.
We assume that a digital chroma signal of a third scanning line is input while continuously inputting digital chroma signals included in scanning lines through input terminal IN. Then, the signal having passed through first delay device 1 is a digital chroma signal of a second scanning line which precedes the third scanning line. The signal having passed through second delay device 2 is a digital chroma signal of a first scanning line which is delayed for two delaying periods. Therefore, the digital chroma signals of three scanning lines are arranged in parallel on the same time axis.
The signals supplied to adder 3 are the digital chroma signals of the first and third scanning lines, which are the same, since one of the chroma signals is alternately transmitted every other line in the MUSE system.
First multiplexer 4 receives the digital chroma signals of the first and third scanning lines summed in adder 3 as one input signal, and the digital chroma signal of the second scanning line output from first delay device 1 as the other input signal. In the meantime, second multiplexer 5 receives the digital chroma signal of the second scanning line output from first delay device as one input signal, and the digital chroma signals of the first and third scanning lines summed in adder 3 as the other input signal. As a result, first and second multiplexers 4 and 5 receive the output signals of first delay device 1 and adder 3, and then respectively generate selected signals in response to line-alternating signal LAS functioning as a control signal. That is, when the signal of either input A of two inputs A and B of first and second multiplexers 4 and 5 is selected in accordance with line-alternating signal LAS, first multiplexer 4 outputs the resultant sum of the digital chroma signals of the first and third scanning lines, and second multiplexer 5 outputs the digital chroma signal of the second scanning line.
As a result, the digital signals of the first and third scanning lines (which precede and succeed the second scanning line, respectively) which are not transmitted together with that of the second scanning line appear at the same time when the digital chroma signal of the present second scanning line is being output.
However, the above-described construction has a problem in that, since the interpolation is performed using the digital chroma signals of a preceding and a succeeding lines, the obtained chroma signal is dissimilar to the original color difference signal having been transmitted in a transmitter side, thereby degrading fidelity.